Differential antinociceptive effects of spinal opioids on foot withdrawal responses evoked by C fibre or Aδ nociceptor activation

1. Intrathecal application of μ, δ, and κ opioids attenuate responses on several tests of animal nociception. However, the potency of these opioids differ depending on which tests were used. One factor contributing to these discrepancies is that different types of noxious stimuli activate different sets of nociceptor types, which may be differentially sensitive to opiate inhibition. To examine this hypothesis, we used a recently developed behavioural test which allows for differential assessment of nociception evoked by the activation of myelinated (Aδ) and unmyelinated C thermonociceptors. 2. Administration of a κ-selective agonist was ineffective on either type of response. δ(1) drugs were slightly more potent on C fibre-mediated responses than on Aδ-mediated responses. 3. Intrathecal μ and δ(2) drugs were antinociceptive on both Aδ and C nociceptor-mediated responses. However, unlike the δ(1) effects, the dose-response curves for μ and δ(2) drugs were significantly more steep for Aδ than for C fibre-mediated responses, potentially indicating differences in the mechanisms by which the drugs act on these 2 response types